Aspirin Titration (Weak Acid / Strong Base) Simulation

General Aim

Determination of acetyl salicylic acid concentration.

Method

Determination of concentration of the active ingredient (acetyl salicylic acid) abundant in aspirin through back acid base titration.

Theoretical Background

In this lab, students will analyze aspirin (acetylsalicylic acid) tablets “analgesic tablets”. Acetylsalicylic acid content will be assessed using a volumetric analysis technique which is called titration. Aspirin tablets contains some inactive components such as binders e.g. starch that are added to hold tablet together during formulation, disintegrating agents e.g. cyclodextrin that are added to help in tablet break down after administration, and lubricants e.g. lactose. All of these are in addition to the active component which is acetyl salicylic acid. Sodium hydroxide could react with the acetylsalicylic acid but not with the other inactive ingredients such as starch binders, etc. according to the following reaction:

C₉H₈O₄ + NaOH → NaC₉H₇O₄+H₂O

This is an acid-base reaction in which the acetylsalicylic acid reacts with the base sodium hydroxide to produce the salt sodium acetylsalicylate and water (acid + base → salt + water).
However, this experiment is preferably carried out as typical indirect or back titration. Standardized NaOH will be used to back titrate an aspirin solution and determine the concentration of aspirin in a typical analgesic tablet. Briefly, known excess NaOH will be added to known amount of aspirin, then the unreacted NaOH after completion of the reaction is back titrated with standardized HCl.
Many reactions such as reaction between acetyl salicylic acid and sodium hydroxide are slow or present unfavorable equilibria for direct titration. Since, aspirin is a weak acid, therefore, it undergoes slow hydrolysis. As shown in the following Figure, each aspirin molecule reacts with two hydroxide ions. To overcome this problem, a known excess amount of base is added to the sample solution and an HCl titration is carried out to determine the amount of unreacted base. This is subtracted from the initial amount of base to find the amount of base that actually reacted with the aspirin and hence the quantity of aspirin in the analyte.

Since this is considered acid-base titration, phenolphthalein is an indicator as it changes color when all the unreacted NaOH has been reacted. This is called the “endpoint” of the reaction. The endpoint is detected when the color of phenolphthalein changes from pink to colorless.

 

 

Principle Of Work

Determination of concentration of the active ingredient (acetyl salicylic acid) abundant in aspirin through back acid-base titration, where excess NaOH is added to aspirin solution to neutralize acetyl salicylic acid. Then, the remaining unreacted NaOH can be back titrated against standardized HCl.